Coal refuse piles are located throughout the United States and pose a significant environmental problem. The piles usually contain 30 to 70 percent coal and exhibit a significant tendency towards spontaneous combustion. In addition to the hazard of uncontrolled fire in a coal refuse pile, the resulting sulphurcontaining smoke is a significant air pollutant. Additionally, acidic drainage from coal refuse piles poses water pollution problems. The increasing number of coal refuse piles continually contributes to such adverse environmental problems. For example, it has been estimated that there are more than 2,000 coal refuse piles, each containing several million tons of refuse, in the Appalachian states alone.
Thus, a need exists for reducing the tendency towards spontaneous combustion in and preventing air and water pollution from coal refuse piles, preferably at minimal cost. In the past, these purposes have been achieved by the relocation and use of coal refuse piles as fill. The coal refuse has been combined with layers of impervious material in order to prevent spontaneous combustion and acid mine drainage. However, this method is disadvantageous owing to the relatively high expense of the impervious material and the required excavation and transport of the pile materials.
In another type of processing, the coal refuse piles have been excavated and transported into generic coal cleaning plants which employ conventional devices such as preferential agglomeration, cyclones, hydrocyclones, float-sink apparatus and the like. For example, in the convertol process, about 7 to 10 percent oil is mixed with finely pulverized coal refuse suspended in water. The water wets the coal but not the inorganic ash precursors. The wetted coal agglomerates on mild stirring and settles out for recovery as pellets while the ash material stays suspended for removal by decantation. While this process has been successfully used in the past, its desirability has been significantly reduced owing to the prohibitive cost of the oil employed therein. Similarly, the use of froth floatation for coal recovery from waste piles has been proposed but the cost of the necessary surfactants is disadvantageous. Additionally, these and similar processes performed in coal cleaning plants are disadvantageous for several further reasons. For example, costly excavation of the coal refuse piles is required. Additionally, large amounts of non-coal material contained in the piles are subjected to expensive transport and handling. Thus, the methods presently known in the art for treating coal refuse piles are not particularly suitable for widespread use.
The Seitzer U.S. Pat. No. 3,723,079 discloses a process for stabilizing dry lignitic and subbituminous coal against spontaneous combustion. The process comprises treating dried coal at a temperature of about 175 to about 225.degree. C. with oxygen in an amount of from about 0.5 percent to about 8 percent by weight of the coal and rehydrating the oxygen-treated coal with water. The Fuchs U.S. Pat. No. 2,338,634 similarly discloses a method for the oxidation of coal wherein the coal is subjected to the action of an oxygen-containing gas at a first elevated temperature and then is heated in contact with air or another oxygen-containing gas at a lower temperature. The second heating stage may employ an oxidation catalyst, for example, ammonium nitrate, potassium nitrate, oxides and salts of chromium, manganese, vanadium, iron, cobalt and nickel, or bases such as potassium, sodium and ammonium hydroxides, and organic bases.
The Schulz et al U.S. Pat. No. 4,305,728 discloses coal suspensions which are prepared from water and the product resulting from the reaction of coal and a base such as, for example, sodium, potassium or calcium hydroxide. The coal suspensions are used in transporting the coal. The Lahart U.S. Pat. No. 1,555,590 discloses a method for treating coal which increases the completeness of combustion and results in the development of greater heat energy. The process comprises mixing the coal with common salt, unslacked lime and Portland cement. British Patent No. 505,729 discloses a further process for producing solid fuels and motor fuels by affecting the oxidation of coal extracts. The extracts are contacted with a stream of air or oxygen-containing gases at raised temperatures, for example 80.degree. C. to 250.degree. C. Finally, European Patent Application No. 237,122 and Chemical Abstracts, Vol. 109, 230324g (1988) and 39641d (1988) disclose methods for treating aqueous solutions of alkaline salts of humic acids. Calcium hydroxide is disclosed as a precipitating agent.
However, none of these prior art references teaches a method for treating coal refuse piles and a substantial need exists for providing such a method which reduces the environmental hazards associated with such materials.